Hydrocarbons are chemical compounds which consist exclusively of carbon and hydrogen. Alkenes (synonym: olefins) are hydrocarbons which have a C═C double bond in the molecule. Alkanes (synonym: paraffins), on the other hand, are hydrocarbons which have only single bonds. They are therefore also referred to as saturated.
In organic chemistry, hydrocarbons are frequently designated according to the number of carbon atoms which they have per molecule, in that the respective class of substances is preceded by the prefix Cn. “n” is the respective number of carbon atoms in a molecule. Thus, C4 olefins are substances from the class of alkenes having four carbon atoms. C8 olefins correspondingly have eight carbon atoms per molecule. Where the prefix Cn+ is used hereinafter, it refers to a class of substances which have more than n carbon atoms per molecule. A C4+ olefin accordingly has at least five carbon atoms.
The simplest olefin is ethene (ethylene). It has two carbon atoms. Ethene is an important commodity chemical and is therefore produced in large quantities. This is usually effected by cracking of naphtha. In addition, it can be obtained by dehydrogenation of ethane, which in turn is a constituent of natural gas. Owing to the increasing exploitation of unconventional sources of natural gas and decreasing recovery of petroleum, the proportion of ethene based on natural gas is steadily increasing. The physical properties of ethene and the production thereof are described in:                Zimmermann, Heinz and Walzl, Roland: Ethylene. Ullmann's Encyclopedia of Industrial Chemistry (2009).        
Oligomerization is understood to mean the reaction of hydrocarbons with themselves to form correspondingly longer-chain hydrocarbons, the so-called oligomers. Olefins having from two to eight carbon atoms can be oligomerized quite efficiently.
Thus, for example, olefins having six carbon atoms (hexene) can be formed by oligomerization of two olefins having three carbon atoms. The oligomerization of two molecules with one another is also referred to as dimerization. If, in contrast, three olefins having three carbon atoms are joined to one another (trimerization), the result is an olefin having nine carbon atoms. The tetramerization of ethene results in octenes, i.e. olefins having eight carbon atoms
It should be noted that oligomerization always results in a mixture of different oligomers (oligomerizate). Thus the oligomerization of ethane forms not only ethene dimers but also, in parallel, trimers and tetramers. The oligomerizate thus comprises a range of C2+ olefins of different lengths.
The oligomers of one carbon atom count are moreover present in different isomeric structures: thus the dimerization of ethene forms the three isomeric C4 olefins 1-butene, cis-2-butene and trans-2-butene, while the trimerization of ethene can form up to ten different C6 olefin isomers, namely 1-hexene, (E)-2-hexene, (Z)-2-hexene, (E)-3-hexene, (Z)-3-hexene, (R)-3-methyl-1-pentene, (S)-3-methyl-1-pentene, (E)-3-methyl-2-pentene, (Z)-3-methyl-2-pentene and 2-ethyl-1-butene. The variety of C8 olefins formed by C2 tetramerization is even greater.
It can therefore be stated that oligomerization of ethene forms a highly complex mixture of different olefins.
However, only the substances 1-butene and 1-hexene, which are used as monomers or comonomers in the production of plastics, are of industrial interest. Olefins having eight carbon atoms may be converted by hydroformylation and hydrogenation into C9 alcohols which in turn serve as starting materials for the production of a very wide variety of PVC plasticizers.
Those oligomerizing ethene are thus faced with the problem of removing the less sought-after substances from the oligomerizate.
Provided that olefins having different numbers of carbon atoms are to be separated this is readily possible by thermal separation methods (distillation) since the different chain lengths result in widely spaced boiling points. However, there is no such hard separation criterion for mixtures of isomers having identical carbon atom counts because the boiling points of the isomers are usually very close together. Distillative separation is then possible only with great apparatus complexity and high energy energy use, thus resulting in a separation that is far too costly.
Thus for example the boiling point of 1-hexene is 63° C. and that of isohexene (correct name: 4-methyl-1-pentene) is 54° C. This small difference in the boiling point makes it uneconomic to obtain the two pure substances 1-hexene and isohexene from a mixture by distillation; even more so when it is not a binary mixture of these two substances that is concerned but rather an oligomerizate having a multiplicity of further C6 olefin isomers whose respective boiling points are likewise within this narrow temperature window.
US2010/0099934A1 employs a trick to economically separate a mixture comprising isohexene and 1-hexene: the C6 olefin mixture is subjected to an etherification step where the isohexene is reacted with methanol to afford ether.
The etherification converts markedly more isohexene than 1-hexene. This affords a reaction mixture which comprises not only 1-hexene but also the ether 3-methyl-3-methoxypentane in place of the isohexene. Since 3-methyl-3-methoxypentane—also known as methyl tert-hexyl ether (MTHxE)—has a higher boiling point than 1-hexene, the 1-hexene can be removed from the MTHxE by distillation at low cost and complexity. In this way, selective isohexene etherification makes it possible to obtain pure 1-hexene from C6 olefin mixtures such as are generated in ethene oligomerization.
With regard to the process described in US2010/0099934A1, the question of what happens to the removed MTHxE remains. An obvious option is to feed the ether mixture into the fuel pool as an anti-knock agent, similarly to the teaching in U.S. Pat. No. 5,752,992. The carbon atoms bound therein are thus ultimately burned and are no longer available as a starting material for high-value specialty chemicals.
It should moreover be noted that ethene oligomerization does not provide any isohexene (4-methyl-1-pentene) at all but rather only the ten C6 olefins listed above. The technical problem of separating 1-hexene and isohexene solved in US2010/0099934A1 consequently does not arise at all in the workup of ethene oligomerizates; on the contrary it must be solved when C6 olefin mixtures originating from metathesis of C4 olefins are to be utilized.